Conventionally, an object position estimation device, an object position estimation method and a program are utilized in order to estimate position (i.e. position to a floor plane) of target objects in image monitoring systems. A technology in relation to this kind of object position estimation device is disclosed in the Japanese Patent Application Laid-Open No. 2008-15573 (henceforth, referred to as “pertinent art”). In the pertinent art, it disclosed an invention of a device which determines existence of target objects in a real space using a plurality of camera images, and estimates position of the target objects. Following to a method of a device which estimates position of a target object according to the pertinent art, it arranges a plurality of cameras so that field of view of each camera overlaps each other in a real space (i.e. three dimensional spaces) where the position estimation is performed. Then, a moving object area of the target object is extracted from image acquired from each camera by using an image processing including a background subtraction method and a frame difference method.
Each camera is in a state in which the camera calibration is performed in advance so that the coordinate of the camera image of the two dimensions and the real space of the three dimensions can be mutually converted. Each camera captures the target object at this state. Next, a device according to the pertinent art reversely projects the image including moving object areas extracted from each camera to a plane where it is designated in the real space. Next, the device according to the pertinent art detects an overlap of the reversely projected moving object areas from all cameras on each plane. In this way, the device according to the pertinent art determines existence of the moving object areas (target objects) in the real space and estimates position of the objects.